As we move into the summer months, let's take a look at some previous summers that contained some similar climate characteristics and environmental conditions. Latest indications are that the El Nino that had begun weakening during the winter, has eroded to the point, that conditions are now generally back to a Neutral state, if not weak La Nina. As the summer unfolds, however, latest model indications are still mixed with their sea surface temperatures /SST/ projections for the summer and beyond. A few models are predicting temperatures to continue to cool, indicating a La Nina pattern, while others are indicating little change (a Neutral pattern) for the summer and yet, a few are even indicating the pattern reverting back to the weak El Nino. The actual final outcome of this summers Pacific SST, while generally not as important to our resultant weather as in the wintertime, may still have enough of an affect on the downwind circulation over the country to make a notable difference in this summer's outcome. The Climate Prediction Center states the following about the SST model predictions as of mid May:
"The latest statistical and coupled model forecasts show a large spread in the forecasts for the next several months. While some indicate the possibility that La Ni¤a will develop during the second half of 2003, others indicate a resurgence of El Ni¤o conditions by the end of the year. However, based on current conditions and recent observed trends, it appears likely that cold episode (La Ni¤a) conditions will develop in the tropical Pacific during the next few months".
Which way the Pacific SST do ultimately trend may, more or less, influence our summer weather here in the Great Lakes and Southeast Michigan. Since the Pacific water temperatures are in a state of flux, careful consideration must be made in regard to this changing phrase evolution and any possible consequences on our summer weather. With the expected La Nina phase development during the summer and the overall weaker climatic patterns, the CPC has chosen Equal Chances /EC/ for both temperatures and rainfall. This means there are equal chances of either: above normal, normal, or below normal temperatures and rainfall for this summer.
When researching for this local outlook, along with the CPC Pacific SST data and predictions, the summers that followed the winter seasons previously studied for the "Winter 2002-03 Outlook" were used. In addition, the overall preceding winter and spring weather trends (upper wind flow and surface) were considered when evaluating these summers in the study. While solar flare activity continues to be on the wane, other items of importance such as the trends of the Eastern Pacific Oscillation /EPO/ and the North Atlantic Oscillation /NAO/ (including the Arctic Oscillation /AO/) are taken into account. Since the global upper wind patterns retract northward and weaken during the summer, their effects on our weather are notably weaker when compared to the their winter counterparts. Finally, the overall recent spring weather pattern was compared to the springs of 1973 and 1978, since their proceeding winters had the most in common with this past winter in the winter study.
The EPO was primarily negative during much of the winter and while the NAO averaged closer to a neutral phase, the Arctic Oscillation /AO/ (part of the NAO) was mainly negative for the balance of the winter. The predominantly negative EPO and AO were mainly responsible for our past cold winter and cool spring. Despite the seasonal change and weakening of the upper wind pattern during the past spring, the EPO and NAO have basically continued their familiar dominant negative phases into early June. In addition, one must remember there also has been, overall, an impressive and rather persistent northwesterly flow into the region since last October. As a result of this northwest flow, every month in Detroit since October has had temperatures average below normal except one, April, and by just +0.3 of a degree, so basically a normal month. This type of extensive cool stretch has not been seen in extreme Southeast Lower Michigan since 1979, when eight months in a row averaged below normal. Even as the upper wind pattern seasonally retreats northward toward the North Pole with the onset of Summer, the Arctic Oscillation /AO/ has remained fairly aggressive in diving cool pools of air into the eastern half of the country. In conjunction with the AO, the Eastern Pacific Oscillation /EPO/ frequently continues to build an upper level ridge of high pressure along the West Coast, helping to deliver the below normal temperatures downstream into our region.
It was discussed in the "Reflecting Back on the Winter Outlook and Spring Trends" write up in May that this past winter strongly resembled the Winter's of 1973 and 1978 with temperatures mainly hovering between normal and below throughout the winter. Snowfall was also heavier than normal and was mainly concentrated over extreme Southeast Lower Michigan. The similar patterns of 1973 and 1978 diverged more later in the spring and summer, and offer little in the way of a consistent trend for the summer. In 1973, the summer averaged just slightly above normal and was wet, while the Summer of '78 was cool and dry. Recently, however, this past April and May have resembled the weather and upper air patterns of 1973 more so than 1978.
During the recent spring, the overall pattern continued to remain cool with a relative increase in precipitation that resulted in mainly near normal amounts or rain, for a change. So, in a sense, the recent spring surface pattern continued that of the winter, with of course, weaker versions of storm tracks and cold outbreaks. The southern stream storm track, along with cool upper low pressure systems diving out of Canada, continued to be rather active and far south right into early June. Along with that, some impressive cold high pressure systems continued their descent southeast across Canada, through the Great Lakes and to the East Coast. Therefore, the combination of these "left-over" trends resulted in the below normal temperatures and much needed rainfall this spring especially in May, whose precipitation was above to well above normal. There evidently has been enough rainfall this spring to dampen the previous surface drought conditions over Southeast Lower Michigan. In fact, the Drought Outlook at the opening of June for this summer has improved considerably for the Great Lakes. Updated versions of this outlook and current drought conditions can be checked throughout the summer.
The following Table-1 represents the summers researched (all data is from the Detroit records which has the largest data base).
Table-1 ___________________________________________________________________________ --------------------------------------------------------------------------- Summer Detroit June July August ___________________________________________________________________________ 1971-2000 Temp/Pcpn Temp/Pcpn Temp/Pcpn Temp/Pcpn 30 Year Normals- 71.4 / 9.81 69.0 / 3.55 73.5 / 3.16 71.8 / 3.10 100 Year Average- 71.0 / 9.50 (approx) ___________________________________________________________________________ Summer Temp/Pcpn of 1882 - 69.4 / 9.78 66.6 / 3.70 70.3 / 1.65 71.4 / 4.43 1886 - 70.1 / 8.67 67.9 / 2.45 71.6 / 2.02 70.8 / 4.20 1914 - 70.7 / 8.89 68.0 / 3.25 72.7 / 1.80 71.4 / 3.84 1924 - 68.7 / 5.64 -12- 65.6 / 3.15 69.9 / 0.78 70.7 / 1.71 1932 + 72.3 / 8.48 70.3 / 1.35 73.0 / 3.11 73.7 / 4.02 1940 +/- 71.4 /11.95 67.6 / 3.27 73.4 / 1.17 73.1 / 7.51 1954 +/- 71.1 / 7.05 71.3 / 2.97 71.8 / 2.01 70.3 / 2.07 1964 +/- 71.6 /10.59 69.6 / 2.35 76.0 / 2.37 69.1 / 5.87 1973 +/- 71.8 /11.19 69.9 / 4.86 72.6 / 4.66 72.9 / 1.67 1978 - 69.8 / 6.39 -20- 66.8 / 2.69 70.6 / 1.97 71.9 / 1.73 1988 -3- + 74.2 / 6.53 70.4 / 0.97 77.1 / 2.43 75.1 / 3.13 1992 (2) - 67.0 /10.76 65.5 / 2.35 68.8 / 5.91 66.7 / 2.50 1995 -1- + 74.5 / 8.66 71.6 / 1.55 74.7 / 3.40 77.1 / 3.71 ---------------------------------------------------------------------------- Mean: 71.0 / 8.81 68.5 / 2.68 72.5 / 2.56 71.9 / 3.57 ( ) Coldest or Wettest Ranking - - Warmest or Driest Ranking + above normal - below normal +/- normal (within .5 degree or .50 precipitation) ALL RESEARCHED DATA AND RECENT SPRING TRENDS SUGGEST A NEAR NORMAL SUMMER AS FAR AS TEMPERATURES - AND NORMAL TO ABOVE NORMAL IN REGARD TO RAINFALL
One of the first things that strikes the observer when looking at the above summer data is the dominance of cool to near normal summers. Out of the 13 summers in the study, six averaged below normal and four averaged near normal, therefore, ten of the 13 summers averaged near normal to below. Only three of the 13 had temperatures that produced a warmer than normal summer. Curiosity of why the three summers were warmer (especially when two placed in the 1st and 3rd place for hottest summers) lead to a deeper investigation. Reflecting back to the variability of the predicted Pacific SST for the summer (explained in the first couple of paragraphs) shows some interesting findings in previous summer patterns. According to the Climate Prediction Center /CPC/ during El Nino summers temperatures across the Great Lakes tend to average around normal, while during La Nina summers temperatures tend to average above normal. (both images)
Remember, the Climate Prediction Center spoke of La Nina conditions developing (or increasing) during the summer months. The timing and how strong that La Nina ultimately forms may very well have an influence in how this summer weather evolves. Also, there is generally a certain amount of lag time between that time and its affects on our weather. The summers in this study where El Nino conditions evolved into a La Nina were indeed, the ones that did average near normal to above over the region. These summers include 1954/N/, 1964/N/, 1973/N/ 1988/A/ and 1995/A/.
During a typical El Nino summer, local records indicate summers of near normal to below temperatures across Southeast Lower Michigan. These summers include 1882/B/, 1914/B/, 1940/N/ and 1992/B/. Neutral summers in the group include 1886/B/, 1924/B/ 1932 /A/, 1978 /B/. Generally, Neutral phased summers have very weak affects on our weather, but the ones chosen in this study, at least, (where the trend is, El Nino - Neutral - La Nina), resulted in normal or below temperatures.
It is felt that the weather this summer will be more variable and not so persistently cool (or below normal) the way the past several months have been. Ironically and interesting to note, that the average temperature of all the summers researched fell dead-on the 100 year normal at Detroit /71.0/.
Again, one must remember that summer-time patterns are considerably weaker and resulting weather patterns are more variable and rainfall is mainly of convective origin rather than stratiform. Accordingly to the CPC, La Nina summers, on average, are drier than normal summers, while in El Nino summers, rainfall averages near normal to above. (both images) In the local study, rainfall in eight out of the 13 summers totaled below normal. However, these drier summers occurred in both the cooler and warmer summers. The driest of summers, 1924 /5.64"/ and 1978 /6.39"/, ranked in the top 20 driest summers (1924 -12th driest and 1978 - 20th driest). Both summers turned out to be cooler than normal and were El Nino evolving into Neutral summers.. Rainfall fell in the near normal category just one summer, 1882, and fell above normal in four summers, 1940, 1964, 1973 and 1992 (and in all but 1992, the temperatures averaged near normal). Looking at just this data, one could say there would be a substantial argument for below normal rainfall this summer. After all, eight out of 13 dry summers leaves a 62 % chance for a dry summer.
However, recent trends have supported above normal snowfall along with a wetter spring as the dominant storm track still favors the Northern Ohio Valley (wet conditions have dominated across the Ohio Valley) . As a summer pattern establishes itself, it is likely this pattern will retreat further north into the Great Lakes (similar to that of 1973). This would support rainfall across the region to continue to more generous (but more scattered) into the summer months...unlike the past few summers. This, along with more intermittent and weaker northwest upper flow (that would still supply the occasional clashing of air masses) and this would point to a normal, if not locally above normal, rainfall scenario
While most of the past several months have averaged cooler than normal due to an aggressive northwesterly flow, the summer's average temperature is still expected to fall within the normal range. The local data above also suggests the best chances of normal to above normal temperatures will come mid to late summer. Generally, the coolest weather (or best chances for below normal temperature departures) was in June or in some cases, even into early July, where the dominant cool upper flow persisted the longest. The warmest weather (or best chances for above normal temperature departures) was generally seen in July and/or August. Still, this summer is not expected to be as warm as last summer nor contain as many ninety degree days. Last year, 25 ninety degree days were recorded (normally we see about 12).
As far as rainfall is concerned, while the majority of summers in the study proved to have below normal rainfall, recent persistent storm tracks and overall upper wind patterns, may prove to supply the region with normal (to possibly above normal rainfall in some areas) this summer.
Here's to good weather whenever and where ever your vacation falls this summer.
Check back in the early fall for this summer's write up!